Field of the Invention
The present invention relates generally to a Doppler-effect speed sensing apparatus for detecting and determining the ground speed of a motor vehicle with respect to the ground or road surface, by utilizing the Doppler effect or Doppler shift, and more particularly to a technique for improving the accuracy of determination of abnormality or erroneous nature of the detected ground speed.
In the field of controlling an automotive vehicle, there has been a high need for precisely detecting the ground speed of the vehicle relative to the ground surface. An example of a known Doppler radar speed detecting apparatus which satisfies the above need includes (a) a transmitter which generates and transmits a wave towards the ground surface, (b) a receiver which receives a portion of the wave reflected by the ground surface, and (c) output means for producing an output indicative of the vehicle ground speed, on the basis of the frequency of the wave as transmitted from the transmitter (hereinafter referred to as "transmitting frequency" and the frequency of the wave as received by the receiver (hereinafter referred to as "receiving frequency"), and according to the principle of the well known "Doppler effect" or "Doppler shift". Namely, such a Doppler radar speed sensor produces an output based on a shift of the receiving frequency relative to the transmitting frequency.
Generally, a Doppler-effect vehicle ground speed sensor permits higher accuracy of detection of the vehicle running speed, than a conventional speed estimating device which is adapted to estimate the vehicle running speed from the speeds of the vehicle wheels. On the other hand, the Doppler speed sensor is subject to an influence by some operating conditions, to which the speed estimating device is immune. For example, the speed detecting accuracy of the Doppler speed sensor is adversely affected by the condition of the road or ground surface from which the transmitted wave is reflected. Further, a member through which the wave from the transmitter is transmitted towards the ground surface, and a member through which the wave reflected by the ground surface is received by the receiver are likely to be stained or soiled by foreign matters, whereby the magnitude of the output signal of the receiver tends to be undesirably lowered.
In the light of the above drawback, it has already been proposed to provide means for monitoring the accuracy of detection or measurement of the vehicle ground speed by such a Doppler-effect ground speed detecting apparatus, that is, means for determining whether the output of the apparatus is sufficiently accurate, or abnormal or erroneous with an excessive error introduced therein. According to the proposed Doppler-effect detecting apparatus, the estimated vehicle speed obtained by the speed estimating device is used as the ground speed, with or without some modification or compensation if the sensor output is found erroneous. This proposed approach is effective to minimize the reduction in the detecting accuracy of the Doppler-effect detecting apparatus. An example of the proposed detecting apparatus is disclosed in JP-A-63-46961 (published in 1988), wherein abnormality detecting means is provided, in addition to the transmitter, receiver and output means as described above. Described more specifically, the abnormality detecting means is adapted to determine if a difference between the ground speed as detected by the detecting apparatus and the estimated vehicle speed as estimated by the speed estimating device is larger than a predetermined threshold for more than a predetermined length of time. If so, the abnormality detecting means determines that there exists some abnormal condition in the detecting apparatus. The abnormality detecting means finds no abnormality if the above difference is not larger than the threshold for more than the predetermined period.
The abnormality detecting means of the above proposed Doppler-effect ground speed detecting apparatus operates on an assumption that since the accuracy of the estimated vehicle speed is generally high, the vehicle ground speed is substantially equal to the estimated vehicle speed as long as the Doppler-effect ground speed detecting apparatus is normal or intact. Actually, however, the accuracy of the estimated vehicle ground speed based on the wheel speeds is not sufficiently stable. For instance, where the brake pressures for all of the four wheels of a 4-wheel drive vehicle are regulated in an anti-lock fashion, the braking torques applied to each pair of wheels connected through a differential gear device interfere with each other, whereby the vehicle ground speed cannot be accurately estimated on the basis of the wheel speeds. Similarly, the accuracy of estimation of the vehicle ground speed tends to be lowered when any one of the tires is changed to a spare tire which has a smaller diameter than the other tires. For the above reasons, the conventional abnormality detecting device which relies solely on the difference between the estimated vehicle speed and the detected vehicle ground speed is not capable of finding or detecting abnormality or erroneous nature of the detected vehicle ground speed with sufficiently high accuracy.